Hey mama I used to fortify my babies breastmilk with coconut oil. I would put in a certain amount per oz (can't remember now) and she loved it. She did not like formula, and I did not want to throw away BM because it sat out too long with formula in it. Formula can also contribute to the constipation. I'm still trying to pack pounds on my girl. She's tiny and eats like a bird. It's so stressful. Good luck!

I second the coconut oil!
We had my DD on a high fat/high calorie diet when she was around 9-10 months old. She actually used to eat coconut oil off of a spoon -- she loved it! I would offer her a few spoonfuls after she drank her milk; it really helped a lot (in keeping her weight steady).
Also, pureed prunes are calorie dense and are great for preventing constipation!

Our dr told us to put our lo one a high caloric diet as at the time he was being ffomrula fed. so for every 3 ounces of formula we had to put 5 scoops for formula in it. Aside from the other suggestions mamas gave as far as food this might be an option for his milk. THough our baby had trouble with cosnitpation too. We tryope dth edark karo syrup int he bottle and miralz helped a little to per dr. But what i thought worked the best was glyrcein (we put some in his little num to help him poop). I wasnt a big fan and thot we really shoudl have to do this every couple days but the Gastro said it was better thatn the miralax bcuz it was just glycein...becasue with the myralax we were jut putting somethign else in to his system when he already had enough and enough probs pooping. Good luck mama.